NtLRP1, a tobacco leucine-rich repeat gene with a possible role as a modulator of the hypersensitive response

Silencing a Simple Extracellular Leucine-Rich Repeat Gene OsI-BAK1 Enhances the Resistance of Rice to Brown Planthopper Nilaparvata lugens

Many plant proteins with extracellular leucine-rich repeat (eLRR) domains play an important role in plant immunity. However, the role of one class of eLRR plant proteins—the simple eLRR proteins—in plant defenses against herbivores remains largely unknown. Here, we found that a simple eLRR protein OsI-BAK1 in rice localizes to the plasma membrane. Its expression was induced by mechanical wounding, the infestation of gravid females of brown planthopper (BPH) Nilaparvata lugens or white-backed planthopper Sogatella furcifera and treatment with methyl jasmonate or abscisic acid. Silencing OsI-BAK1 (ir-ibak1) in rice enhanced the BPH-induced transcript levels of three defense-related WRKY genes (OsWRKY24, OsWRKY53 and OsWRKY70) but decreased the induced levels of ethylene. Bioassays revealed that the hatching rate was significantly lower in BPH eggs laid on ir-ibak1 plants than wild-type (WT) plants; moreover, gravid BPH females preferred to oviposit on WT plants over ir-ibak1 plants. The exogenous application of ethephon on ir-ibak1 plants eliminated the BPH oviposition preference between WT and ir-ibak1 plants but had no effect on the hatching rate of BPH eggs. These findings suggest that OsI-BAK1 acts as a negative modulator of defense responses in rice to BPH and that BPH might exploit this modulator for its own benefit.

Geminivirus C4 antagonizes the HIR1-mediated hypersensitive response by inhibiting the HIR1 self-interaction and promoting degradation of the protein

Tomato leaf curl Yunnan virus (TLCYnV)-encoded C4 protein induces the upregulation of the hypersensitive induced reaction 1 (HIR1) gene but interferes with the HIR1-mediated hypersensitive response (HR). HIR1 self-interaction is essential for the HIR1-induced HR. TLCYnV C4 impairs the HIR1 self-interaction and concomitantly increases the amount of Leucine-Rich Repeat protein 1 (LRR1), a modulator of HIR1, which binds to HIR1. LRR1 promotes the degradation of HIR1, compromising the HIR1-mediated HR. This study provides new insights into the mechanisms employed by a viral protein to counter host resistance through the cooption of the host regulatory system.

Meta-analysis of drought-tolerant genotypes in Oryza sativa: A network-based approach

BACKGROUND

Drought is a severe environmental stress. It is estimated that about 50% of the world rice production is affected mainly by drought. Apart from conventional breeding strategies to develop drought-tolerant crops, innovative computational approaches may provide insights into the underlying molecular mechanisms of stress response and identify drought-responsive markers. Here we propose a network-based computational approach involving a meta-analytic study of seven drought-tolerant rice genotypes under drought stress.

RESULTS

Co-expression networks enable large-scale analysis of gene-pair associations and tightly coupled clusters that may represent coordinated biological processes. Considering differentially expressed genes in the co-expressed modules and supplementing external information such as resistance/tolerance QTLs, transcription factors, network-based topological measures, we identify and prioritize drought-adaptive co-expressed gene modules and potential candidate genes. Using the candidate genes that are well-represented across the datasets as 'seed' genes, two drought-specific protein-protein interaction networks (PPINs) are constructed with up- and down-regulated genes. Cluster analysis of the up-regulated PPIN revealed ABA signalling pathway as a central process in drought response with a probable crosstalk with energy metabolic processes. Tightly coupled gene clusters representing up-regulation of core cellular respiratory processes and enhanced degradation of branched chain amino acids and cell wall metabolism are identified. Cluster analysis of down-regulated PPIN provides a snapshot of major processes associated with photosynthesis, growth, development and protein synthesis, most of which are shut down during drought. Differential regulation of phytohormones, e.g., jasmonic acid, cell wall metabolism, signalling and posttranslational modifications associated with biotic stress are elucidated. Functional characterization of topologically important, drought-responsive uncharacterized genes that may play a role in important processes such as ABA signalling, calcium signalling, photosynthesis and cell wall metabolism is discussed. Further transgenic studies on these genes may help in elucidating their biological role under stress conditions.

CONCLUSION

Currently, a large number of resources for rice functional genomics exist which are mostly underutilized by the scientific community. In this study, a computational approach integrating information from various resources such as gene co-expression networks, protein-protein interactions and pathway-level information is proposed to provide a systems-level view of complex drought-responsive processes across the drought-tolerant genotypes.

A novel leucine-rich repeat protein, CaLRR51, acts as a positive regulator in the response of pepper to Ralstonia solanacearum infection

The leucine-rich repeat (LRR) proteins play important roles in the recognition of corresponding ligands and signal transduction networks in plant defence responses. Herein, a novel LRR protein from Capsicum annuum, CaLRR51, was identified and characterized. It was localized to the plasma membrane and transcriptionally up-regulated by Ralstonia solanacearum infection (RSI), as well as the exogenous application of salicylic acid (SA), jasmonic acid (JA) and ethephon (ETH). Virus-induced gene silencing of CaLRR51 significantly increased the susceptibility of pepper to RSI. By contrast, transient overexpression of CaLRR51 in pepper plants activated hypersensitive response (HR)-like cell death, and up-regulated the defence-related marker genes, including PO2, HIR1, PR1, DEF1 and ACO1. Moreover, ectopic overexpression of CaLRR51 in transgenic tobacco plants significantly enhanced the resistance to RSI. Transcriptional expression of the corresponding defence-related marker genes in transgenic tobacco plants was also found to be enhanced by the overexpression of CaLRR51, which was potentiated by RSI. These loss- and gain-of-function assays suggest that CaLRR51 acts as a positive regulator in the response of pepper to RSI. In addition, the putative signal peptide and transmembrane region were found to be required for plasma membrane targeting of CaLRR51, which is indispensable for the role of CaLRR51 in plant immunity.

Functional roles of the pepper leucine-rich repeat protein and its interactions with pathogenesis-related and hypersensitive-induced proteins in plant cell death and immunity

Pepper leucine-rich repeat protein (CaLRR1) interacts with defense response proteins to regulate plant cell death and immunity. This review highlights the current understanding of the molecular functions of CaLRR1 and its interactor proteins. Plant cell death and immune responses to microbial pathogens are controlled by complex and tightly regulated molecular signaling networks. Xanthomonas campestris pv. vesicatoria (Xcv)-inducible pepper (Capsicum annuum) leucine-rich repeat protein 1 (CaLRR1) serves as a molecular marker for plant cell death and immunity signaling. In this review, we discuss recent advances in elucidating the functional roles of CaLRR1 and its interacting plant proteins, and understanding how they are involved in the cell death and defense responses. CaLRR1 physically interacts with pepper pathogenesis-related proteins (CaPR10 and CaPR4b) and hypersensitive-induced reaction protein (CaHIR1) to regulate plant cell death and defense responses. CaLRR1 is produced in the cytoplasm and trafficked to the extracellular matrix. CaLRR1 binds to CaPR10 in the cytoplasm and CaPR4b and CaHIR1 at the plasma membrane. CaLRR1 synergistically accelerates CaPR10-triggered hypersensitive cell death, but negatively regulates CaPR4b- and CaHIR1-triggered cell death. CaHIR1 interacts with Xcv filamentous hemagglutinin (Fha1) to trigger disease-associated cell death. The subcellular localization and cellular function of these CaLRR1 interactors during plant cell death and defense responses were elucidated by Agrobacterium-mediated transient expression, virus-induced gene silencing, and transgenic overexpression studies. CaPR10, CaPR4b, and CaHIR1 positively regulate defense signaling mediated by salicylic acid and reactive oxygen species, thereby activating hypersensitive cell death and disease resistance. A comprehensive understanding of the molecular functions of CaLRR1 and its interacting protein partners in cell death and defense responses will provide valuable information for the molecular genetics of plant disease resistance, which could be exploited as a sustainable disease management strategy.

NtRING1, putative RING-finger E3 ligase protein, is a positive regulator of the early stages of elicitin-induced HR in tobacco

KEY MESSAGE

NtRING1 is a RING-finger protein with a putative E3 ligase activity. NtRING1 regulates HR establishment against different pathogens. Loss-/gain-of-function of NtRING1 altered early stages of HR phenotype establishment. Plant defence responses against pathogens often involve the restriction of pathogens by inducing a hypersensitive response (HR). cDNA clones DD11-39, DD38-11 and DD34-26 were previously obtained from a differential screen aimed at characterising tobacco genes with an elicitin-induced HR-specific pattern of expression. Our precedent observations suggested that DD11-39, DD38-11 and DD34-26 might play roles in the HR establishment. Only for DD11-39 a full-length cDNA sequence was obtained and the corresponding protein encoded for a type-HC RING-finger/putative E3 ligase protein which we termed NtRING1. The expression of NtRING1 was upregulated upon HR induction by elicitin, Ralstonia solanacearum, or tobacco mosaic virus (TMV) in tobacco. Silencing of NtRING1 remarkably delayed the establishment of elicitin-induced HR in tobacco as well as the expression of different early induction genes in tissues undergoing HR. Accordingly, transient overexpression of NtRING1 accelerated the HR launching upon elicitin treatment. Taking together, our data suggests that NtRING1 plays a functional role in the early establishment of HR.

Pathogenesis-related protein 4b interacts with leucine-rich repeat protein 1 to suppress PR4b-triggered cell death and defense response in pepper

To control defense and cell-death signaling, plants contain an abundance of pathogen recognition receptors such as leucine-rich repeat (LRR) proteins. Here we show that pepper (Capsicum annuum) LRR1 interacts with the pepper pathogenesis-related (PR) protein 4b, PR4b, in yeast and in planta. PR4b is synthesized in the endoplasmic reticulum, interacts with LRR1 in the plasma membrane, and is secreted to the apoplast via the plasma membrane. Binding of PR4b to LRR1 requires the chitin-binding domain of PR4b. Purified PR4b protein inhibits spore germination and mycelial growth of plant fungal pathogens. Transient expression of PR4b triggers hypersensitive cell death. This cell death is compromised by co-expression of LRR1 as a negative regulator in Nicotiana benthamiana leaves. LRR1/PR4b silencing in pepper and PR4b over-expression in Arabidopsis thaliana demonstrated that LRR1 and PR4b are necessary for defense responses to Pseudomonas syringae pv. tomato and Hyaloperonospora arabidopsidis (Hpa) infection. The mutant of the PR4b Arabidopsis ortholog, pr4, showed enhanced susceptibility to Hpa infection. Together, our results suggest that PR4b functions as a positive modulator of plant cell death and defense responses. However, the activity of PR4b is suppressed by interaction with LRR1.

Sorghum extracellular leucine-rich repeat protein SbLRR2 mediates lead tolerance in transgenic Arabidopsis

A sorghum pathogen-inducible gene predicted to encode a simple extracellular leucine-rich repeat (LRR) protein SbLRR2 was previously isolated. LRR was the only domain identified in SbLRR2 and its homologous sequences. Phylogenetic analysis revealed that they are distinct from the simple extracellular LRR proteins reported previously. Agrobacterium-mediated transient expression in tobacco leaf cells demonstrated that the SbLRR2-EYFP (enhanced yellow fluorescent protein) fusion protein was targeted to the extracellular space. Transgenic analysis of SbLRR2 revealed its role in enhancing lead [Pb(II)] tolerance in Arabidopsis. Consequently, SbLRR2-overexpressing lines were found to show alleviated Pb(II)-induced root inhibition, lower levels of Pb(II) accumulation and enhanced transcription of AtPDR12 which encodes a plasma membrane ATP-bind cassette (ABC)-type transporter formerly shown to contribute to Pb(II) detoxification. However, all the Pb(II) tolerance responses were abolished when SbLRR2 was overexpressed in an atpdr12 T-DNA insertion line. The extracellular localization of SbLRR2 was also shown to be essential for the Pb(II) phenotypes and AtPDR12 up-regulation. Taken together, SbLRR2 appears to mediate Pb(II) tolerance through the elevation of AtPDR12 expression in transgenic Arabidopsis, thus activating a glutathione-independent mechanism for detoxification. Further investigations revealed the Pb(II)-induced transcriptional activation of SbLRR2 and several highly conserved AtPDR12 homologs in sorghum seedlings, suggesting the possibility of a common molecular mechanism for Pb(II) tolerance in diverse plant species.

Simultaneous transcriptome analysis of Sorghum and Bipolaris sorghicola by using RNA-seq in combination with de novo transcriptome assembly

The recent development of RNA sequencing (RNA-seq) technology has enabled us to analyze the transcriptomes of plants and their pathogens simultaneously. However, RNA-seq often relies on aligning reads to a reference genome and is thus unsuitable for analyzing most plant pathogens, as their genomes have not been fully sequenced. Here, we analyzed the transcriptomes of Sorghum bicolor (L.) Moench and its pathogen Bipolaris sorghicola simultaneously by using RNA-seq in combination with de novo transcriptome assembly. We sequenced the mixed transcriptome of the disease-resistant sorghum cultivar SIL-05 and B. sorghicola in infected leaves in the early stages of infection (12 and 24 h post-inoculation) by using Illumina mRNA-Seq technology. Sorghum gene expression was quantified by aligning reads to the sorghum reference genome. For B. sorghicola, reads that could not be aligned to the sorghum reference genome were subjected to de novo transcriptome assembly. We identified genes of B. sorghicola for growth of this fungus in sorghum, as well as genes in sorghum for the defense response. The genes of B. sorghicola included those encoding Woronin body major protein, LysM domain-containing intracellular hyphae protein, transcriptional factors CpcA and HacA, and plant cell-wall degrading enzymes. The sorghum genes included those encoding two receptors of the simple eLRR domain protein family, transcription factors that are putative orthologs of OsWRKY45 and OsWRKY28 in rice, and a class III peroxidase that is a homolog involved in disease resistance in the Poaceae. These defense-related genes were particularly strongly induced among paralogs annotated in the sorghum genome. Thus, in the absence of genome sequences for the pathogen, simultaneous transcriptome analysis of plant and pathogen by using de novo assembly was useful for identifying putative key genes in the plant-pathogen interaction.

Functional analysis and expressional characterization of rice ankyrin repeat-containing protein, OsPIANK1, in basal defense against Magnaporthe oryzae attack

The ankyrin repeat-containing protein gene OsPIANK1 (AK068021) in rice (Oryza sativa L.) was previously shown to be upregulated following infection with the rice leaf blight pathogen Xanthomonas oryzae pv oryzae (Xoo). In this study, we further characterized the role of OsPIANK1 in basal defense against Magnaporthe oryzae (M.oryzae) by 5' deletion analysis of its promoter and overexpression of the gene. The promoter of OsPIANK1 with 1,985 bps in length was sufficient to induce the OsPIANK1 response to inoculation with M.oryzae and to exogenous application of methyl jasmonate (MeJA) or salicylic acid (SA), but not to exogenous application of abscisic acid (ABA). A TCA-element present in the region between -563 bp and -249 bp may be responsible for the OsPIANK1 response to both M.oryzae infection and exogenous SA application. The JERE box, CGTCA-box, and two MYB binding sites locating in the region between -1985 bp and -907 bp may be responsible for the response of OsPIANK1 to exogenous MeJA. OsPIANK1 expression was upregulated after inoculation with M.oryzae and after treatment with exogenous SA and MeJA. Overexpression of OsPIANK1 enhanced resistance of rice to M.oryzae, although it did not confer complete resistance. The enhanced resistance to M.oryzae was accompanied by enhanced transcriptional expression of SA- and JA-dependent genes such as NH1, WKRY13, PAL, AOS2, PR1b, and PR5. This evidence suggests that OsPIANK1 acted as a positive regulator in rice basal defense mediated by SA- and JA-signaling pathways.

The role of G-proteins in plant immunity

Heterotrimeric G-proteins play an important regulatory role in multiple physiological processes, including the plant immune response, and substantial progress has been made in elucidating the G-protein-mediated defense-signaling network. This mini-review discusses the importance of G-proteins in plant immunity. We also provide an overview of how G-proteins affect plant cell death and stomatal movement. Our recent studies demonstrated that G-proteins are involved in signal transduction and induction of stomatal closure and defense responses. We also discuss future directions for G-protein signaling studies involving plant immunity.

Over-expression of rice leucine-rich repeat protein results in activation of defense response, thereby enhancing resistance to bacterial soft rot in Chinese cabbage

Pectobacterium carotovorum subsp. carotovorum causes soft rot disease in various plants, including Chinese cabbage. The simple extracellular leucine-rich repeat (eLRR) domain proteins have been implicated in disease resistance. Rice leucine-rich repeat protein (OsLRP), a rice simple eLRR domain protein, is induced by pathogens, phytohormones, and salt. To see whether OsLRP enhances disease resistance to bacterial soft rot, OsLRP was introduced into Chinese cabbage by Agrobacterium-mediated transformation. Two independent transgenic lines over-expressing OsLRP were generated and further analyzed. Transgenic lines over-expressing OsLRP showed enhanced disease resistance to bacterial soft rot compared to non-transgenic control. Bacterial growth was retarded in transgenic lines over-expressing OsLRP compared to non-transgenic controls. We propose that OsLRP confers enhanced resistance to bacterial soft rot. Monitoring expression of defense-associated genes in transgenic lines over-expressing OsLRP, two different glucanases and Brassica rapa polygalacturonase inhibiting protein 2, PDF1 were constitutively activated in transgenic lines compared to non-transgenic control. Taken together, heterologous expression of OsLRP results in the activation of defense response and enhanced resistance to bacterial soft rot.

The hypersensitive induced reaction and leucine-rich repeat proteins regulate plant cell death associated with disease and plant immunity

Pathogen-induced programmed cell death (PCD) is intimately linked with disease resistance and susceptibility. However, the molecular components regulating PCD, including hypersensitive and susceptible cell death, are largely unknown in plants. In this study, we show that pathogen-induced Capsicum annuum hypersensitive induced reaction 1 (CaHIR1) and leucine-rich repeat 1 (CaLRR1) function as distinct plant PCD regulators in pepper plants during Xanthomonas campestris pv. vesicatoria infection. Confocal microscopy and protein gel blot analyses revealed that CaLRR1 and CaHIR1 localize to the extracellular matrix and plasma membrane (PM), respectively. Bimolecular fluorescent complementation and coimmunoprecipitation assays showed that the extracellular CaLRR1 specifically binds to the PM-located CaHIR1 in pepper leaves. Overexpression of CaHIR1 triggered pathogen-independent cell death in pepper and Nicotiana benthamiana plants but not in yeast cells. Virus-induced gene silencing (VIGS) of CaLRR1 and CaHIR1 distinctly strengthened and compromised hypersensitive and susceptible cell death in pepper plants, respectively. Endogenous salicylic acid levels and pathogenesis-related gene transcripts were elevated in CaHIR1-silenced plants. VIGS of NbLRR1 and NbHIR1, the N. benthamiana orthologs of CaLRR1 and CaHIR1, regulated Bax- and avrPto-/Pto-induced PCD. Taken together, these results suggest that leucine-rich repeat and hypersensitive induced reaction proteins may act as cell-death regulators associated with plant immunity and disease.

Devil inside: does plant programmed cell death involve the endomembrane system?

Eukaryotic cells have to constantly cope with environmental cues and integrate developmental signals. Cell survival or death is the only possible outcome. In the field of animal biology, tremendous efforts have been put into the understanding of mechanisms underlying cell fate decision. Distinct organelles have been proven to sense a broad range of stimuli and, if necessary, engage cell death signalling pathway(s). Over the years, forward and reverse genetic screens have uncovered numerous regulators of programmed cell death (PCD) in plants. However, to date, molecular networks are far from being deciphered and, apart from the autophagic compartment, no organelles have been assigned a clear role in the regulation of cellular suicide. The endomembrane system (ES) seems, nevertheless, to harbour a significant number of cell death mediators. In this review, the involvement of this system in the control of plant PCD is discussed in-depth, as well as compared and contrasted with what is known in animal and yeast systems.

A novel simple extracellular leucine-rich repeat (eLRR) domain protein from rice (OsLRR1) enters the endosomal pathway and interacts with the hypersensitive-induced reaction protein 1 (OsHIR1)

Receptor-like protein kinases (RLKs) containing an extracellular leucine-rich repeat (eLRR) domain, a transmembrane domain and a cytoplasmic kinase domain play important roles in plant disease resistance. Simple eLRR domain proteins structurally resembling the extracellular portion of the RLKs may also participate in signalling transduction and plant defence response. Yet the molecular mechanisms and subcellular localization in regulating plant disease resistance of these simple eLRR domain proteins are still largely unclear. We provided the first experimental evidence to demonstrate the subcellular localization and trafficking of a novel simple eLRR domain protein (OsLRR1) in the endosomal pathway, using both confocal and electron microscopy. Yeast two-hybrid and in vitro pull-down assays show that OsLRR1 interacts with the rice hypersensitive-induced response protein 1 (OsHIR1) which is localized on plasma membrane. The interaction between LRR1 and HIR1 homologs was shown to be highly conserved among different plant species, suggesting a close functional relationship between the two proteins. The function of OsLRR1 in plant defence response was examined by gain-of-function tests using transgenic Arabidopsis thaliana. The protective effects of OsLRR1 against bacterial pathogen infection were shown by the alleviating of disease symptoms, lowering of pathogen titres and higher expression of defence marker genes.

The LCB2 subunit of the sphingolip biosynthesis enzyme serine palmitoyltransferase can function as an attenuator of the hypersensitive response and Bax-induced cell death

Previous results showed that expression of the gene encoding the LONG-CHAIN BASE2 (LCB(2)) subunit of serine palmitoyltransferase (SPT), designated BcLCB(2), from nonheading Chinese cabbage (Brassica campestris ssp. chinensis) was up-regulated during hypersensitive cell death (HCD) induced by the Phytophthora boehmeriae elicitor PB90. Overexpression of BcLCB(2) in Nicotiana tabacum leaves suppressed the HCD normally initiated by elicitors and PB90-triggered H(2)O(2) accumulation. BcLCB(2) also functioned as a suppressor of mouse Bcl-2 associated X (Bax) protein-mediated HCD and cell death caused by Ralstonia solanacearum. BcLCB(2) overexpression suppressed Bax- and oxidant stress-triggered yeast cell death. Reactive oxygen species (ROS) accumulation induced by Bax was compromised in BcLCB(2)-overexpressing yeast cells. The findings that NbLCB(2) silencing in Nicotiana benthamiana enhanced elicitor-triggered HCD, combined with the fact that myriocin, a potent inhibitor of SPT, had no effect on Bax-induced programmed cell death, suggested that suppression of cell death was not involved in the dominant-negative effect that resulted from BcLCB(2) overexpression. A BcLCB(2) mutant assay showed that the suppression was not involved in SPT activity. The results suggest that plant HCD and stress-induced yeast cell death might share a common signal transduction pathway involving LCB(2), and that LCB(2) protects against cell death by inhibiting ROS accumulation, this inhibition being independent of SPT activity.

The leucine-rich repeat (LRR) protein, CaLRR1, interacts with the hypersensitive induced reaction (HIR) protein, CaHIR1, and suppresses cell death induced by the CaHIR1 protein

Leucine-rich repeat proteins (LRRs) function in a number of signal transduction pathways via protein-protein interactions. The gene encoding a small protein of pepper, CaLRR1, is specifically induced upon pathogen challenge and treatment with pathogen-associated molecular patterns (PAMPs). We identified a pepper hypersensitive induced reaction (CaHIR1) protein that interacts with the LRR domain of the CaLRR1 protein using yeast two-hybrid screening. Ectopic expression of the pepper CaHIR1 gene induces cell death in tobacco and Arabidopsis, indicating that the CaHIR1 protein may be a positive regulator of HR-like cell death. Because transformation is very difficult in pepper plants, we over-expressed CaLRR1 and CaHIR1 in Arabidopsis to determine cellular functions of the two genes. The over-expression of the CaHIR1 gene, but not the CaLRR1 gene, in transgenic Arabidopsis confers disease resistance in response to Pseudomonas syringae infection, accompanied by the strong expression of PR genes, the accumulation of both salicylic acid and H(2)O(2), and K(+) efflux in plant cells. In Arabidopsis and tobacco plants over-expressing both CaHIR1 and CaLRR1, the CaLRR1 protein suppresses not only CaHIR1-induced cell death, but also PR gene expression elicited by CaHIR1 via its association with HIR protein. We propose that the CaLRR1 protein functions as a novel negative regulator of CaHIR1-mediated cell death responses in plants.